A relativistically broadened iron line from an Accreting Millisecond Pulsar

Abstract

The capabilities of XMM-Newton have been fully exploited to detect a broadened iron Kα emission line from the 2.5 ms Accreting Millisecond Pulsar, SAX J1808.4-3658. The energy of the transition is compatible with fluorescence from neutral/lowly ionized iron. The observed large width (FWHM more than 1 keV) can be explained through Doppler and relativistic broadening from the inner rings of an accretion disc close to the NS. From a fit of the line shape with a diskline model we obtain an estimate of the inner disc radius of 18.0-5.6+7.6km for a 1.4 M⊙ neutron star. The disc is therefore truncated inside the corotation radius (31 km for SAX J1808.4-3658), in agreement with the observation of coherent pulsations. From our estimate of the inner disc radius, we infer that the magnetic field of the neutron star is in the range 1-5×108G. © 2010 American Institute of Physics.